"it wouldn't be able to completely depopulate Russia immediately. "
"Russia" has never been the target for the UK "independent" nuclear deterrent.
Moscow, and in particular the Kremlin is the main target. Any warheads left other will be assigned to high value targets to disable the Russian's ability to fight a war.
To be a deterrent with such a small force both the UK and France have to hurt their opponents in a way that makes them think twice about starting a war in the first place.